The geography of the region about Devil's lake and the Dallas of the Wisconsin: with some notes on its surface geology

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Page 73 - ... only one set of conditions is altered. If, instead of favoring advance, the two series of conditions conspire to cause the ice to recede, the recession will likewise be greater than when but one set of conditions is favorable thereto. Greenland affords an example of the conditions here described. The large part of the half million or more square miles which this body of land is estimated to contain, is covered by a vast sheet of snow and ice, thousands of feet in thickness. In this field of snow...
Page 158 - But during this time a visit was made, for the purpose of comparative study, to the copper-mining region of Keweenaw point. The method followed in the season's field work was to visit all parts of the two areas above mentioned which were known to have rock outcrops, or in which outcrops might be reasonably expected. In such parts north and south traverses were made at intervals of one-half mile, or more frequently when occasion demanded. Thus, in the region where outcrops occur frequently, the north...
Page 72 - It was long since recognized that the materials of the drift did not originate where they now lie, and that, in consequence, they sustain no genetic relationship to the strata on which they rest.
Page 123 - On the basis of position stratified drift deposits may be classified as follows: 1. Extraglacial deposits, made by the waters of any glacial epoch if they flowed and deposited beyond the farthest limit of the ice. 2. Supermorainic deposits made chiefly during the final retreat of the ice from the locality where they occur, but sometimes by extraglacial streams or lakes of a much later time.
Page 82 - ... variation. This amounts in the aggregate to hundreds of feet. The continental ice sheet, therefore, modified the topography of the region it covered, not only by the wear it effected, but also by the deposits it made. In some places it chanced that the greater thicknesses of drift were left in the position formerly marked by valleys.
Page 78 - This accumulation of earthy and stony debris in the bottom of the ice, gave it a rough and grinding lower surface, which enabled it to abrade the land over which it passed much more effectively than ice alone could have done. Every hill and every mound which the ice encountered contested its advance. Every sufficiently resistant elevation compelled the ice to pass around or over it ; but even in these cases the ice left its marks upon the surface to which it yielded. The powerful pressure of pure...
Page 46 - ... most effectively. The areas of softer rock would be brought low, and the outcrops of hard rock (A, Fig. 12) would determine the position of the ridges on the surface during all the later stages of a cycle. If there were bodies of hard rock, such as the trap rock in the Triassic formation, the shale on either hand would be worn down much more readily than the trap itself, and the latter would, at a certain stage in a cycle of erosion, stand out prominently. The Piedmont plain is in 77 just that...
Page 120 - When the channels remained constant in position for a long time, the aggradation may have been considerable. In so far as the channel deposits were made near the edge of the ice during the time of its maximum extension or retreat, they were likely to remain undisturbed during its melting, after which they stood out Fio.
Page 72 - Other things remaining constant, therefore, increase of pressure will cause the ice-sheet to extend itself further from the center of motion. Increase of snowfall will increase the pressure of the snow- and ice-field by increasing its mass. If, therefore, the precipitation over a given snow-field be increased for a period of years, the ice-sheet's marginal motion will be accelerated and its area enlarged. A decrease of precipitation, taken in connection with unchanged wastage, would decrease the^...
Page 163 - The CopperBearing Rocks of Lake Superior" ; US Geol. Survey, Monograph V., pp. 234-259, 1883. Moses Strong. "Geology of the Upper St. Croix District" (edited by TC Chamberlin) ; Geol. of Wis., vol. Ill, pp. 363428, 1880. ET Sweet. "Geology of the Western Lake Superior District" ; Geol. of Wis., vol. Ill, pp. 303-362, 1880. The report by Strong deals especially with the St. Croix range; that by Sweet with the Douglas range and gives an account of the earlier explorations for this metal. These reports...

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